Extension instance gets multiple manifestations when cython is present

Originally reported by: jaraco (Bitbucket: jaraco, GitHub: jaraco)

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In this comment, @johnyf details an issue where the presence of Cython in the Python environment triggers a failure due to multiple manifestations of distutils.Extension or setuptools.Extension.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/488

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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I believe the issue here is related to how setuptools.sandbox removes itself from sys.modules in order to allow another version to be used (such as when upgrading setuptools).

I suspect that because setuptools imports cython in buildext, it should also remove it when hiding setuptools and for the same reasons.

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Also hide Cython when hiding setuptools, as setuptools will have imported Cython and Cython references the distutils.Extension. Fixes #488.

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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@johnyf Would you please test with 19.6b1 and confirm it addresses the issue?

Original comment by johnyf (Bitbucket: johnyf, GitHub: johnyf):

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As of 19.6b1 the issue appears to have been resolved, while it is present in 19.5.
Tested with Cython == 0.23.4, psutil == 3.4.2 and two packages:

1. the minimal example described in 309
2. dd with psutil in install_requires.

Thank you very much for the fix.

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Glad to hear it. 19.6 is released now.